netdev - Re: [PATCH net-next v3] net: ip: avoid OOM kills with large UDP sends over loopback

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Date:   Wed, 23 Jun 2021 20:11:33 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Jakub Kicinski <kuba@...nel.org>, davem@...emloft.net
Cc:     netdev@...r.kernel.org, willemb@...gle.com, dsahern@...il.com,
        yoshfuji@...ux-ipv6.org, Dave Jones <dsj@...com>
Subject: Re: [PATCH net-next v3] net: ip: avoid OOM kills with large UDP sends
 over loopback



On 6/23/21 8:05 PM, Eric Dumazet wrote:
> 
> 
> On 6/23/21 6:23 PM, Jakub Kicinski wrote:
>> Dave observed number of machines hitting OOM on the UDP send
>> path. The workload seems to be sending large UDP packets over
>> loopback. Since loopback has MTU of 64k kernel will try to
>> allocate an skb with up to 64k of head space. This has a good
>> chance of failing under memory pressure. What's worse if
>> the message length is <32k the allocation may trigger an
>> OOM killer.
>>
>> This is entirely avoidable, we can use an skb with page frags.
>>
>> af_unix solves a similar problem by limiting the head
>> length to SKB_MAX_ALLOC. This seems like a good and simple
>> approach. It means that UDP messages > 16kB will now
>> use fragments if underlying device supports SG, if extra
>> allocator pressure causes regressions in real workloads
>> we can switch to trying the large allocation first and
>> falling back.
>>
>> Reported-by: Dave Jones <dsj@...com>
>> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
>> ---
> 
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> Thanks !
> 

I am taking this back.

IPv6 side also needs to account for sizeof(struct frag_hdr) ?